Making particles with radial variation

ABSTRACT

Provided is a process of making polymeric beads comprising
         (a) providing a suspension of monomer droplets in an aqueous medium at pH of 7 or less,
           wherein the monomer droplets comprise one or more monofunctional vinyl monomers, one or more multifunctional vinyl monomers, and one or more initiators,   wherein the aqueous medium comprises one or more derivatives of a nitrite salt in an amount, by weight based on the weight of the aqueous medium, of 0.005% to 0.5%,   
           (b) initiating polymerization of the monomer,   wherein no pH-raising substance is added after beginning step (b) until 60% or more by weight of all monofunctional monomer has been converted to polymer.

Polymeric ion exchange resins in the form of beads are used in manyapplications, including, for example, as chelating resins or as anionexchangers or as cation exchangers. In many of these applications, theresins are subjected to stresses which can cause substantial breakage ofthe beads. These stresses may be mechanical, for example due to crushingor collisions between beads or between the beads and their containers;or may be osmotic, such as when the beads are subjected to sudden and/orrepeated changes in electrolyte concentration. The breakage of the ionexchange beads in a column causes one or more of the following problems:substantial losses in the efficiency of the column; rise in pressuredrop through the column; and contamination of the product stream. Any ofthese problems will cause large costs in replacing the broken resins.

Ion exchange resins are either gel type or macroporous type. In somecases, gel type (microporous) resins have poor osmotic properties. As aresult, macroporous type resins are generally employed when good osmoticproperties are essential. However, macroporous resins often have poormechanical properties and often have substantially lower ion exchangecapacity than gel type resins. It would be desirable to provide a geltype ion exchange resin having one or more of the following: goodosmotic properties, good mechanical properties, and high ion exchangecapacity.

One approach to making gel type resins is described in EP 0 101 943,which describes methods in which a polymeric matrix that containsradicals is contacted with a monomer feed to make core/shell beads. Thisis a complex process in which a radical-containing polymer must be made,and then monomer must be fed into the container that has theradical-containing polymer. Additionally, the process described in EP 0101 943 is most advantageous when the concentration of polymerized unitsof multifunctional vinyl monomer in the shell is lower than theconcentration of polymerized units of multifunctional vinyl monomer inthe core. It would be desirable to provide high-strength gel type resinswith a process that did not require a separate monomer-feeding step. Itwould also be desirable to provide a high-strength gel type resin inwhich the concentration of polymerized units of multifunctional vinylmonomer was the same or nearly the same throughout the resin.

The following is a statement of the invention.

A first aspect of the present invention is a process of making polymericbeads comprising

-   (a) providing a suspension of monomer droplets in an aqueous medium    at pH of 7 or less,    -   wherein the monomer droplets comprise one or more monofunctional        vinyl monomers, one or more multifunctional vinyl monomers, and        one or more initiators,    -   wherein the aqueous medium comprises one or more derivatives of        a nitrite salt in an amount, by weight based on the weight of        the aqueous medium, of 0.005% to 0.5%, and-   (b) initiating polymerization of the monomer,    -   wherein no pH-raising substance is added after beginning        step (b) until 60% or more by    -   weight of all monofunctional monomer has been converted to        polymer.

A second aspect of the present invention is a polymeric bead havingradius R wherein the polymer comprises 0.3% to 20% by weight, based onthe weight of the polymer, of polymerized units of one or moremultifunctional vinyl monomer and 80% to 99.7% by weight, based on theweight of the polymer, of polymerized units of one or moremonofunctional vinyl monomer,

-   (a) wherein the polymerized units of multifunctional vinyl monomer    have radial distribution factor MR of 0.9 to 1.1, wherein    MR=CMSHELL/CMCORE, wherein CMSHELL is the average concentration of    polymerized units of multifunctional vinyl monomer located at a    distance from the center of the bead of 0.8*R to R, and wherein    CMCORE is the average concentration of polymerized units of    multifunctional vinyl monomer located at a distance from the center    of the bead of 0 to 0.5*R, and-   (b) wherein some of the vinyl groups in the polymerized units of    multivinyl monomer are unreacted, and the unreacted vinyl groups    have a radial distribution factor VR of 2.5 or higher, wherein VR is    determined by a Raman spectroscopic measurement performed on the    bead, wherein    VR=V1SHELL/V1CORE,    wherein V1SHELL is the average of ratio V1 for measurements made at    a distance from the center of the bead of 0.8*R to R, wherein V1CORE    is the average of ratio V1 for measurements made at a distance from    the center of the bead of 0 to 0.5*R, wherein V1=PCC/PAR, wherein    PCC is the height of the Raman spectroscopic peak due to stretching    of carbon-carbon double bonds, and PAR is the height of the Raman    spectroscopic reference peak due to stretching of the aromatic ring    at 1000 cm⁻¹.

The following is a brief description of the drawings. FIG. 1 shows across section of a polymeric bead. FIG. 2 shows a vertical cross sectionof apparatus for measuring Raman spectroscopy on a cross section of aswollen polymeric bead. FIG. 3 shows the Raman spectroscopic analysis ofa polymeric particle according to comparative Example 1B. FIG. 4 showsthe Raman spectroscopic analysis of a polymeric particle according toExample 4A.

The following is a detailed description of the invention.

As used herein, the following terms have the designated definitions,unless the context clearly indicates otherwise.

A “polymer,” as used herein is a relatively large molecule made up ofthe reaction products of smaller chemical repeat units. Polymers mayhave structures that are linear, branched, star shaped, looped,hyperbranched, crosslinked, or a combination thereof; polymers may havea single type of repeat unit (“homopolymers”) or they may have more thanone type of repeat unit (“copolymers”). Copolymers may have the varioustypes of repeat units arranged randomly, in sequence, in blocks, inother arrangements, or in any mixture or combination thereof.

Molecules that can react with each other to form the repeat units of apolymer are known herein as “monomers.” The repeat units so formed areknown herein as “polymerized units” of the monomer.

Vinyl monomers have the structure

where each of R¹, R², R³, and R⁴ is, independently, a hydrogen, ahalogen, an aliphatic group (such as, for example, an alkyl group), asubstituted aliphatic group, an aryl group, a substituted aryl group,another substituted or unsubstituted organic group, or any combinationthereof. Vinyl monomers are capable of free radical polymerization toform polymers. Some vinyl monomers have one or more polymerizablecarbon-carbon double bonds incorporated into one or more of R¹, R², R³,and R⁴; such vinyl monomers are known herein as multifunctional vinylmonomers. Vinyl monomers with exactly one polymerizable carbon-carbondouble bond are known herein as monofunctional vinyl monomers.

Styrenic monomers are vinyl monomers in which each of R¹ and R² ishydrogen, R³ is hydrogen or alkyl, and —R⁴ has the structure

where each of R⁵, R⁶, R⁷, R⁸, and R⁹ is, independently, a hydrogen, ahalogen, an aliphatic group (such as, for example, an alkyl group or avinyl group), a substituted aliphatic group, an aryl group, asubstituted aryl group, another substituted or unsubstituted organicgroup, or any combination thereof.

Acrylic monomers are vinyl monomers in which each of R¹ and R² ishydrogen; R³ is either hydrogen or methyl; and —R⁴ has one of thefollowing structures:

where each of R¹¹, R¹², and R¹⁴ is, independently, hydrogen, a C₁ to C₁₄alkyl group, or a substituted C₁ to C₁₄ alkyl group.

A reaction among monomers to form one or more polymers is referred toherein as a polymerization process. A polymerization process is saidherein to have run to completion when the amount of unreacted monomer inthe vessel in which the polymerization process is taking place is 5 mass% or less, based on the sum of the mass of unreacted monomer and themass of polymer made in the polymerization process.

As used herein, an inhibitor is a molecule that interacts with a freeradical to create a moiety (herein the “dead-end” moiety) that is notsusceptible to free radical polymerization. The inhibitor may interactwith a free radical to form the dead-end moiety directly, or theinhibitor may first form one or more intermediates, and the intermediatemay interact with a radical to form a dead-end moiety. In cases wherethe inhibitor first forms an intermediate, the formation of theintermediate may occur through a reaction between the inhibitor and afree radical.

As used herein, an initiator is a molecule that is stable at ambientconditions but that is capable under certain conditions of producing oneor more fragments that bears a free radical, and that fragment iscapable of interacting with a monomer to start a free radicalpolymerization process. The conditions that cause production of afragment bearing a free radical include, for example, elevatedtemperature, participation in an oxidation-reduction reaction, exposureto ultraviolet and/or ionizing radiation, or a combination thereof.

A porogen is a compound that is soluble in the monomer or mixture ofmonomers used in the practice of the present invention. That is, at 25°C., 100 grams or more of porogen will dissolve in 100 grams of monomeror mixture of monomers used in the practice of the present invention.The polymer does not imbibe large amounts of porogen. That is, at 25°C., the polymer formed in the practice of the present invention imbibes5 grams or less of porogen per 100 grams of polymer.

Macroporous polymeric beads have a porous structure with average porediameter of 20 nm or larger. Pore diameter is measured using theBrunauer-Emmett-Teller (BET) method using nitrogen gas. Macroporouspolymeric beads are normally made by incorporating a porogen intomonomer droplets. The porogen is soluble in the monomer, but the polymerdoes not dissolve in the porogen, so that as the polymer forms,phase-separated domains of porogen remain. After polymerization, theporogen is removed by evaporation or by washing with solvent. The porousstructure of the polymeric bead is the empty space left when the porogenis removed from its phase-separated domains.

Gel type polymeric beads are made without the use of porogen. The poresin gel type polymeric beads are the free volumes between the atoms inthe entangled, crosslinked polymer chains of the polymeric bead. Thepores in gel type polymeric beads are smaller than 20 nm. In some cases,the pores in gel type resins are too small to be detected using the BETmethod.

As used herein, ion exchange is a process in which ions in solutionbecome attached to a solid resin (an ion exchange resin), and those ionsare exchanged for ions of the same type of charge that are released bythe ion exchange resin. Functional groups located on the resin haveopposite charge to the ions being exchanged, and those functional groupsare known herein as ion exchange groups.

A compound is said herein to be water-soluble if 5 grams or more of thecompound forms a stable solution in 100 ml of water at 25° C. In thecase of some water-soluble polymers, the water may need to be heatedabove 25° C. in order to make the polymer dissolve, but after cooling to25° C., the solution is stable when held at 25° C.

As used herein, a base compound is a compound that has the ability toaccept a proton to form the conjugate acid of that compound, and theconjugate acid of that compound has pKa of 9 or greater. As used herein,an acid compound is a compound that has the ability to release a proton,and the compound has pKa of 5 or less. A buffer is either (i) a compoundthat has the ability to accept a proton to form the conjugate acid ofthat compound, and the conjugate acid of that compound has pKa of lessthan 9, or (ii) a compound that has the ability to release a proton, andthe compound has pKa of greater than 5.

As used herein, “ambient conditions” means temperature of approximately25° C. and pressure of 1 atmosphere.

A suspension is a composition that has particles of one substancedistributed through a liquid medium. The distributed particles may beliquid or solid; distributed liquid particles are called droplets. Themedium is “aqueous” if the medium contains 90% or more water by weight,based on the weight of the medium. A suspension may or may not bestable. That is, the distributed particles may or may not have atendency to settle to the bottom of the container or to float to the topof the container, and mechanical agitation may or may not be required tokeep the particles distributed in the medium.

A polymeric bead is a particle that contains 90% or more by weight,based on the weight of the particle, organic polymer. A polymeric beadis spherical or nearly spherical. A polymeric bead is characterized byits radius. If the bead is not spherical, the radius of the bead istaken herein to be the radius of a “reference sphere,” which is theimaginary sphere that has the same volume as the bead. Whether aparticle is spherical or not is assessed by the “sphericity,”represented by the Greek letter Ψ. Sphericity is defined by thefollowing formula, based on the three principal axes of the bead, a(longest), b (middle), and c (shortest):

$\Psi = ( \frac{bc}{a^{2}} )^{(^{1/3})}$

As used herein, a polymerization process is a “single-step”polymerization process when monomer and optionally other compounds areplaced into a container, then the polymerization is initiated, andpolymerization proceeds to completeness without the addition of anyfurther monomer after the initiation of polymerization. A single-steppolymerization process is not a seeded process.

As used herein, a suspension polymerization process is a “seeded”process if the process involves a state (S1) in which the monomerdroplets contain 80% or more monomer by weight based on the weight ofthe droplets; in which the monomer droplets are not undergoingpolymerization; and in which the monomer droplets contain polymer in anamount of 1% or more by weight based on the weight of the droplets. In aseeded process, after state (S1), polymerization of the monomer in themonomer droplets is initiated. In a typical seeded process, a suspensionof polymeric particles is provided, then monomer is added to thesuspension, the monomer imbibes into the polymeric particles, and thenpolymerization of monomer is initiated.

Ratios are described herein as follows. For example, if a ratio is saidto be 3:1 or greater, that ratio may be 3:1 or 5:1 or 100:1 but may notbe 2:1. The general statement of this idea is as follows: when a ratiois said herein to be X:1 or greater, it is meant that the ratio is Y:1,where Y is greater than or equal to X. Similarly, for example, if aratio is said to be 15:1 or less, that ratio may be 15:1 or 10:1 or0.1:1 but may not be 20:1. Stated in a general way: when a ratio is saidherein to be W:1 or less, it is meant that the ratio is Z:1, where Z isless than or equal to W.

While the present invention is not limited to any specific theory, it iscontemplated that the process of the present invention produces apolymeric bead that has a relatively constant concentration ofpolymerized units of multifunctional vinyl monomer throughout the volumeof the bead. A molecule of multifunctional vinyl monomer becomes apolymerized unit when one or more of the polymerizable functional groupsparticipates in a polymerization reaction. There are instances in whicha polymerized unit of a multifunctional vinyl monomer retains one ormore unreacted functional group. It is contemplated that in the presentinvention, while the polymerized units of multifunctional vinyl monomerare evenly distributed through the bead, the unreacted functional groupsattached to such polymerized units are more prevalent in the shellportion of the bead than in the core portion. It is considered that thedensity of crosslink points in the polymeric bead is higher in the corethan in the shell, because more of the functional groups of themultifunctional vinyl monomer have reacted in the shell to create morecrosslink points. Thus it is considered that there is a lower density ofcrosslinks in the shell, even though the distribution of polymerizedunits of multifunctional vinyl monomer is approximately the same in thecore and the shell.

The process of the present invention involves monomer droplets thatcontain vinyl monomer and initiator. The monomer droplets optionallyadditionally contain porogen.

It is useful to characterize the sum of the amount of monomer plus theamount of porogen, as a percentage by weight based on the weight of themonomer droplets. Preferably, that sum is 95% or higher; more preferably97% or higher; more preferably 99% or higher.

Preferably, porogen is either absent or, if present, is present inrelatively small amounts. If porogen is present in the monomer droplets,preferably the amount of porogen is limited to an amount, by weightbased on the weight of the monomer droplets, of 10% or less; morepreferably 3% or less; more preferably 1% or less; more preferably 0.3%or less. More preferably, no porogen is present in the monomer droplets.

Preferably, the amount of monomer in the monomer droplets is, by weightbased on the weight of the droplets, 95% or more; more preferably 97% ormore; more preferably 99% or more.

Preferred vinyl monomers are styrenic monomers, acrylic monomers, andmixtures thereof. Preferably, all the monomers used are selected fromstyrenic monomers, acrylic monomers, and mixtures thereof. Morepreferably, all the monomers used are selected from styrenic monomers.The vinyl monomer includes one or more monofunctional vinyl monomer.Preferred monofunctional vinyl monomers are acrylic and styrenicmonofunctional monomers; more preferred are monofunctional styrenicmonomers; more preferred is styrene. The vinyl monomer also includes oneor more multifunctional vinyl monomer. Preferred multifunctional vinylmonomers are multifunctional styrenic monomers; more preferred isdivinyl benzene. As used herein, the term “divinyl benzene” or “DVB”refers to a mixture containing approximately 63% pure chemical DVB byweight and approximately 37% ethylvinyl benzene by weight, possibly withother chemicals in a total amount of 1% or less. Preferably, the amountof vinyl chloride is, by weight based on the total weight of allmonomers, 0 to 0.1%, more preferably 0 to 0.01%; more preferably 0%.

Preferably, the amount of styrenic monomer, by weight based on theweight of all monomers, is 50% or higher; more preferably 75% or higher;more preferably 88% or higher; more preferably 94% or higher; morepreferably 97% or higher; more preferably 100%.

Preferably, the amount of monofunctional vinyl monomer is, by weightbased on the weight of all monomers, 80% or more: more preferably 85% ormore. Preferably, the amount of monofunctional vinyl monomer is, byweight based on the weight of all monomers, 99.7% or less; morepreferably 99% or less; more preferably 98% or less; more preferably 96%or less; more preferably 94% or less; more preferably 92% or less.

Preferably, the amount of multifunctional vinyl monomer is, by weightbased on the weight of all monomers, 0.3% or more; more preferably 1% ormore; more preferably 2% or more; more preferably 4% or more; morepreferably 6% or more; more preferably 8% or more. Preferably, theamount of multifunctional vinyl monomer is, by weight based on theweight of all monomers, 20% or less; more preferably 15% or less.

Preferably, the monomer droplets contain little or no polymer prior toinitiation of polymerization. The amount of polymer is, by weight basedon the weight of the monomer droplets, preferably 1% or less; morepreferably 0.3% or less; more preferably 0.1% or less; more preferablyzero.

The process of the present invention involves a suspension of themonomer droplets in an aqueous medium. Preferably, the total amount ofmonomer, by weight based on the total weight of the suspension, is 5% ormore; more preferably 10% or more; more preferably 15% or more.Preferably, the total amount of monomer, by weight based on the totalweight of the suspension, is 55% or less; more preferably 35% or less;more preferably 30% or less.

The aqueous medium contains one or more dissolved nitrite salts, thederivatives of that nitrite salt, or a combination thereof. A nitritesalt has the formula M(NO₂)_(v), where M is ammonium or an alkali metalcation or an alkaline earth cation, v is 1 when M is ammonium or analkali metal cation, and v is 2 when M is an alkaline earth cation. Itis considered that when a nitrite salt is dissolved in water, thenitrite ion may undergo chemical reactions to form derivatives such as,for example, nitrous acid and/or compounds of the formula N_(x)O_(y).The amount of these derivatives is characterized by the weight of thedissolved salt plus the amount of salt that would have to be dissolvedin order to produce the amount of derivatives that are present in theaqueous medium. The preferred amount of nitrite salt and its derivativesis, by weight based on the weight of the aqueous medium, 0.005% or more;more preferably 0.008% or more; more preferably 0.011% or more; morepreferably 0.014% or more. The preferred amount of nitrite salt and itsderivatives is, by weight based on the weight of the aqueous medium,0.5% or less; more preferably 0.4% or less; more preferably 0.3% orless; more preferably 0.2% or less.

A preferred nitrite salt is sodium nitrite. Preferably the derivativesof nitrite salt are derivatives of sodium nitrite.

The monomer droplets preferably contain one or more initiator. Preferredinitiators have solubility in 100 mL of water at 25° C. of 1 gram orless; more preferably 0.5 gram or less; more preferably 0.2 gram orless; more preferably 0.1 gram or less. Preferred are peroxide andhydroperoxide initiators; more preferred are peroxide initiators; morepreferred are benzoyl peroxide and derivatives thereof; more preferredis benzoyl peroxide. Preferably, the weight ratio of initiator to totalmonomer is 0.001:1 or higher; more preferably 0.002:1 or higher.Preferably, the weight ratio of initiator to total monomer is 0.02:1 orlower; more preferably 0.01:1 or lower; more preferably 0.007:1 orlower.

The suspension preferably contains one or more water-soluble polymer.Preferred water-soluble polymers are water-soluble polyvinyl alcoholpolymers, water-soluble derivatives of cellulose, and mixtures thereof.Among water-soluble derivatives of cellulose, preferred arecarboxymethyl methylcelluloses. Among polyvinyl alcohol polymers,preferred are those with degree of hydrolysis of 80% to 90%. Preferablythe suspension contains one or more water-soluble polyvinyl alcoholpolymers and one or more water-soluble derivatives of cellulose.

When one or more water-soluble polymers are used, preferably the totalamount of water-soluble polymers is, by weight based on the weight ofthe water, 0.02% or higher; more preferably 0.05% or higher; morepreferably 0.1% or higher. When one or more water-soluble polymers areused, preferably the total amount of water-soluble polymers is, byweight based on the weight of the water, 1% or less; more preferably0.5% or less.

Gelatin may or may not be present in the suspension. When gelatin ispresent, the amount is, by weight based on the weight of the water, 2%or less; or 1% or less; or 0.5% or less. Preferred embodiments havelittle or no gelatin. Preferably the amount of gelatin is sufficientlylow that the amount of gelatin is, by weight based on the weight ofwater, 0 to 0.01%; more preferably 0 to 0.001%. More preferably theamount of gelatin is zero.

Prior to the step (b) of initiating polymerization of monomer, the pH ofthe aqueous medium is 7 or lower. Prior to the step (b) of initiatingpolymerization of monomer, the pH of the aqueous medium is preferably 3or higher; more preferably 4 or higher, more preferably 5 or higher;more preferably 5.5 or higher.

While the present invention is not limited to any specific theory ormechanism, the following is contemplated regarding the operation of thepresent invention. It is considered that, when water-soluble nitritesalt is added to the suspension, some or all of the water-solublenitrite salt dissolves in the water in the aqueous medium, and that, atpH of 7 or below, the presence of hydrogen ions creates an equilibriumbetween dissolved nitrite ion and nitrous acid. The nitrous acid isthought to undergo further chemical reactions to form one or more ofnitrogen monoxide or other compounds of formula NxOy, where x is 1 or 2,y is 1 to 5, and when x is 2, y is 1, 3, 4, or 5. In general, it isexpected that the lower the pH, the greater the production of compoundsof formula NxOy, including NO. It is considered that the most likelyNxOy compound to be produced is nitrogen monoxide (NO), possibly incombination with one or more other NxOy compound. A compound of formulaNxOy that is formed when a water soluble nitrite salt is added to thesuspension is considered herein to be a derivative of the water solublenitrite salt. It is contemplated that because NO is a radical species,NO will act as an inhibitor by reacting with a monomer radical or with aradical on a growing polymer chain, thus terminating the polymerizationreaction. It is contemplated that other NxOy compounds may also act asinhibitors.

It is contemplated that the presence of dissolved nitrite ion and theacidic conditions in the aqueous medium create a system in which freshinhibitor molecules are constantly being formed throughout thepolymerization process, as long as the pH is kept below 7. It iscontemplated that benefits similar to those obtained by the presentinvention could also be obtained by gradually adding an inhibitor (suchas, for example, catechol) to the aqueous medium. Such a procedure wouldalso create a system in which fresh inhibitor molecules were constantlyintroduced. Whatever inhibitor is used, it is contemplated that theinhibitor should be partly or fully soluble in water to allow fortransportation through the aqueous medium and should be partly or fullysoluble in the monomer droplet to allow the inhibitor to diffuse intothe monomer droplet, where it could react with a radical and terminatepolymerization.

The pH of the suspension prior to the beginning of polymerization mayoptionally be established by addition of one or more acid to the aqueousmedium. When an acid is added, preferred acids have first pKa of 3 orhigher; more preferably 4 or higher. When an acid is used, any type ofacid may be used; preferred are organic acids. Preferably, no acid isadded to the aqueous medium; that is, it is preferred that theingredients listed above establish a pH in the suspension that is 7 orlower without the addition of acid. Preferably, no buffer is present inthe aqueous medium.

The nature of the step that initiates polymerization depends in part onthe nature of the initiator that is used. For example, when a thermalinitiator is used, initiation conditions involve establishing atemperature above 25° C. that is high enough for a significant fractionof the initiator molecules to decompose to form free radicals. Foranother example, if a photoinitiator is used, initiation conditionsinvolve exposing the initiator to radiation of sufficiently lowwavelength and of sufficiently high intensity for a significant fractionof the initiator molecules to decompose to form free radicals. Foranother example, when the initiator is a redox initiator, initiationconditions involve the presence of sufficiently high concentration ofboth the oxidant and the reductant such that a significant number offree radicals are produced. Preferably, a thermal initiator is used.Preferably, initiation conditions involve temperature of 65° C. orhigher; more preferably 75° C. or higher. That is, preferably thesuspension is provided at a temperature below 40° C., and the initiatorthat is present does not produce significant number of free radicals atthat temperature. Then, preferably, step (b) involves raising thetemperature to initiation conditions.

After step (b), while polymerization is taking place, at any moment, theextent of the free radical polymerization in the vessel that containsthe suspension may be characterized as follows.Extent=100*PM/TMwhere PM is the mass of polymer formed by the free radicalpolymerization process, and TM is the total mass of monomer that hasbeen added to the vessel.

In some embodiments a base or an appropriate buffer may be added to thesuspension during the polymerization. An appropriate buffer is a bufferthat would raise the pH of the suspension. One motivation for adding abase or an appropriate buffer is that it is considered that raising thepH of the suspension may cause some of the derivatives of the nitritesalt to react to re-form the nitrite salt. Because some of thederivatives, especially one or more of the N_(x)O_(y) compounds, areconsidered to inhibit polymerization, raising the pH is considered toremove some inhibitors from the suspension, which is considered to allowthe polymerization to reach completion more quickly. In the practice ofthe present invention, if a base or an appropriate buffer is added, itis not added at any time from the first initialization of polymerizationto the point when the extent of the reaction is at 60% or higher;preferably 70% or higher; preferably 80% or higher. In some embodiments,no base or appropriate buffer is added to the suspension during thepolymerization.

When a base is added, preferred are organic base compounds and inorganicbase compounds. More preferred are inorganic base compounds; morepreferred are alkali hydroxides and ammonium hydroxide; more preferredare alkali hydroxides. Preferably, when base compound is added to thesuspension, the addition is performed by first forming an aqueoussolution of a base compound and then adding that solution to thesuspension. Preferred aqueous solutions have concentration of basecompound, by weight based on the weight of the solution, of 1% or more;more preferably 2% or more; more preferably 5% or more. Preferredaqueous solutions have concentration of base compound, by weight basedon the weight of the solution, of 50% or less; more preferably 25% orless; more preferably 15% or less; more preferably 10% or less.

When an appropriate buffer is added, preferably the buffer is a compoundthat has the ability to accept a proton to form the conjugate acid ofthat compound, and the conjugate acid of that compound has pKa that isless than 9. Preferably, the conjugate acid of that compound has pKathat is 6 or higher; more preferably 7 or higher; more preferably 7.5 orhigher. Some suitable buffers include, for example, TES(2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonicacid); HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid); DIPSO(3-(N,N-Bis[2-hydroxyethyl]amino)-2-hydroxypropanesulfonic acid,N,N-Bis(2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid); TAPSO(2-Hydroxy-3-[tris(hydroxymethyl)methylamino]-1 -propanesulfonic acid,N-[Tris(hydroxymethy)methyl]-3-amino-2-hydroxypropanesulfonic acid);triethanolamine; N-ethyl morpholine; POPSO(2-Hydroxy-3-[tris(hydroxymethyl)methylamino]-1-propanesulfonic acid,N-[Tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropanesulfonic acid”;EPPS, also known as HEPPS(4-(2-Hydroxyethyl)-1-piperazinepropanesulfonic acid,4-(2-Hydroxyethyl)piperazine-1-propanesulfonic acid,N-(2-Hydroxyethyl)piperazine-N′-(3-propanesulfonic acid)); HEPPSO (CASnumber 865856-46-8); TRIS (2-Amino-2-hydroxymethyl-propane-1,3-diol);tricine; glycylglycine; bicene; TAPS(N-[Tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid,[(2-Hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid);morpholine; N-methyldiethanolamine; AMPO(2-amino-2-methyl-1,3-propanediol); diethanolamine.

The suspension may or may not contain boric acid. When boric acid ispresent, the amount may be, by weight based on the weight of water, 2%or less, or 1% or less, or 0.5% or less. Preferably, the suspensioncontains little or no boric acid. Preferably, the amount of boric acidin the suspension is 0 to 0.01% by weight, based on the weight of water;more preferably 0%.

Prior to the beginning of the polymerization process, droplets arepresent in the suspension, and the droplets contain vinyl monomer andinitiator. Preferably the droplets are distributed throughout theaqueous medium, and preferably the composition of the aqueous mediumcontains water in the amount, by weight based on the weight of thecontinuous liquid medium, of 90% or more; more preferably 95% or more;more preferably 97% or more. Compounds dissolved in the water areconsidered to be part of the continuous liquid medium. Preferably, thevolume average particle size of the droplets is 50 μm to 1,500 μm.

The suspension of monomer droplets is subjected to conditions that causethe initiator to initiate polymerization of the monomer. Preferably, theinitiator is a thermal initiator, and the initiation of polymerizationbegins when the suspension is heated to a temperature that is above 40°C. and that is sufficiently high to cause enough initiator to produceenough radicals to initiate the polymerization.

Preferably, the polymerization is a single step polymerization. That is,preferably, either no monomer is added to the suspension after theinitiation of polymerization or, if any monomer is added, the amount ofmonomer added is, by weight based on the weight of all monomers presentin the suspension at the time of initiation of polymerization, 1% orless; more preferably 0.1% or less. More preferably, no monomer is addedto the suspension after the initiation of polymerization.

Preferably, the process of the present invention is not a seededprocess.

The present invention also involves polymeric beads. The polymeric beadsare preferably made by the method of the present invention. Preferably,the polymeric beads contain polymer. Polymeric beads are particles thatare solid at 25° C. and that contain polymer in the amount, by weightbased on the weight of the polymeric particles, of 90% or more; morepreferably 95% or more.

Preferably the polymeric beads have volume average particle diameter of50 μm or larger; more preferably 100 μm or larger; more preferably 200μm or larger; more preferably 400 μm or larger. Preferably the polymericbeads have volume average particle diameter of 1,500 μm or lower; morepreferably 1,000 μm or lower.

Preferred polymers in the polymeric particles are the polymers formed byfree radical polymerization of the preferred vinyl monomers describedabove. Preferably the polymer contains polymerized units of styrenicmonomer in the amount, by weight based on the weight of the polymer, of5% or more; more preferably 25% or more; more preferably 50% or more;more preferably 75% or more; more preferably 85% or more; morepreferably 95% or more. The types of monomers preferred as polymerizedunits of the polymer are the same as those described above as preferredfor use in the polymerization process.

Preferred polymers have polymerized units of multifunctional vinylmonomer in an amount, by weight based on the weight of the polymer, of0.3% or more; more preferably 0.5% or more; more preferably 1% or more;more preferably 2% or more; more preferably 3% or more; more preferably4% or more. Preferred polymers have polymerized units of multifunctionalvinyl monomer in an amount, by weight based on the weight of thepolymer, of 20% or less; more preferably 15% or less.

Preferred polymers have polymerized units of monofunctional vinylmonomer in an amount, by weight based on the weight of the polymer, of99.7% or less; more preferably 99.5% or less; more preferably 99% orless; more preferably 98% or less; more preferably 97% or less; morepreferably 96% or less. Preferred polymers have polymerized units ofmonofunctional vinyl monomer in an amount, by weight based on the weightof the polymer, of 80% or more; more preferably 85% or more.

It is contemplated that, when two or more of the vinyl groups on asingle molecule of a multifunctional vinyl monomer participate inpolymerization reactions, then that molecule forms a crosslink pointbetween polymer chains. When considering the polymerized units ofmultifunctional vinyl monomers in the polymeric bead, it is contemplatedthat, in some but not all of such polymerized units, two or more of thevinyl groups will have participated in polymerization reactions andformed crosslink points. It is expected that sufficient crosslink pointswill have formed that the polymer in the polymeric bead will be acrosslinked polymer. At the same time, it is also contemplated that somepolymerized units of multifunctional vinyl monomer will have one or morevinyl group that is unreacted (that is, a vinyl group that did notparticipate in a polymerization reaction and that is still intact).

The polymer in the polymeric bead has a relatively even distribution ofpolymerized units of multifunctional vinyl monomer between the centralregion of the bead and the outer shell of the bead. This distributionmay be assessed as follows. The radius of the bead is defined as R. Theshell is defined as the region of the bead located at a distance fromthe center of the bead of from 0.8*R to R. The core is defined as theregion of the bead located at a distance from the center of the bead offrom 0 to 0.5*R. The identification of the shell 1, the core 3, and anintermediate region 2 are illustrated in FIG. 1 .

The concentration of polymerized units of multifunctional vinyl monomerlocated in the shell is labeled CMSHELL, and the concentration ofpolymerized units of multifunctional vinyl monomer located in the coreis labeled CMCORE. The radial distribution factor, labeled MR, isdefined as the quotient of CMSHELL divided by CMCORE:MR=CMSHELL/CMCORE

CMCORE and CMSHELL may be characterized by any convenient units, forexample millimole per cubic centimeter. In practice, because thequotient MR is the important quantity, the unit used for CMSHELL andCMCORE is not important, as long as the same unit is used for bothCMSHELL and CMCORE.

The quotient MR is 0.9 or higher; preferably 0.95 or higher. Thequotient MR is 1.1 or lower; more preferably 1.05 or lower.

The polymerized units of multifunctional vinyl monomer are distributedrelatively evenly throughout the polymeric bead. However, in somepolymerized units of multifunctional vinyl monomer, all of the vinylgroups participated in a polymerization reaction during the formation ofthe bead, while in other polymerized units of multifunctional vinylmonomer, at least one vinyl group did participate in a polymerizationreaction during formation of the bead while at least one vinyl groupremained unreacted. The spatial distribution of the unreacted vinylgroups is a characteristic of the present invention.

The distribution of unreacted vinyl groups is characterized as follows.The core and shell of the bead are defined as described above. Arepresentative bead is cut open to reveal a cross section that includesthe center of the bead. Raman spectroscopy is performed on microscopicregions of the bead. PCC is the height of the Raman peak at 1635 cm⁻¹due to stretching of carbon-carbon double bonds. PAR is the height ofthe Raman reference peak at 1000 cm⁻¹ due to stretching of the aromaticring. The quotient V1=PCC/PAR characterizes the prevalence of doublebonds within the polymer. The quantity V1SHELL is the average of V1 inthe shell, and the quantity V1CORE is the average of V1 in the core. Forunreacted carbon-carbon double bonds, the radial distribution factor isVR=V1SHELL/V1CORE. When performing the Raman spectroscopy, the bead isoptionally swollen in a solvent; the solvent is optionally fullydeuterated.

It is considered that the accuracy of the above Raman spectroscopymethod may be optimized by removing any residual unpolymerizedmonofunctional monomer from the polymeric bead, for example removingresidual styrene by washing with acetone and drying the bead to removethe acetone. This washing step is considered especially useful when itis intended to perform the Raman spectroscopy on a bead that is notswollen in a solvent.

VR is 2.5 or higher, preferably 2.7 or higher; more preferably 2.9 orhigher. Preferably, VR is 10 or lower; more preferably 5 or lower.

The polymeric beads preferably have average sphericity of 0.8 or higher;more preferably 0.85 or higher; more preferably 0.9 or higher; morepreferably 0.95 or higher.

Another method of assessing the difference between the core region ofthe bead and the shell region of the bead is as follows. The core, theshell, and the quantity PAR are defined as above. The sample is swollenwith fully deuterated toluene. It is considered that the shell regionhas lower concentration of crosslinks than the core region, andtherefore the shell region is expected to imbibe more solvent in theswelling process. The quantity PCD is the height of the Raman peak at2122 cm−1 due to stretching of carbon-deuterium bonds in the toluene.The quotient V2=PCD/PCH represents the prevalence of the deuteratedtoluene as compared to the polymer.

The quotient V2 can be converted to a mass-per-volume quotient (MPVQ) asfollows. A reference solution of known concentration of linearpolystyrene in deuterated toluene is made. For this reference solution,the mass per volume of deuterated toluene (MPVTOLREF) and the mass pervolume of polystyrene (MPVPSREF) are both known, and the quotient isMPVQREF=MPVTOLREF/MPVPSREF. Also, the quotient for the referencesolution is measured and labeled V2REF. Then, for any particularexperimental sample, the quotient V2 can be converted to MPVQ asfollows:MPVQ=V2*MPVQREF/V2REF

In a bead swollen with deuterated toluene, the average value of MPVQ inthe shell is MPVQSHELL, and the average value of MPVQ in the core isMPVQCORE. Then the radial distribution factor for solvent isRDFS=MPVQSHELL/MPVQCORE. Preferably, RDFS is 2.5 or greater. Preferably,RDFS is 10 or less.

Another method of showing the inhomogeneity of the bead uses nuclearmagnetic resonance (NMR) spectroscopy. The analysis of the NMR resultsare based on the following preliminary observations. When chloroform(CHCl₃) is studied in a pure state, a characteristic chemical shift isobserved in the NMR spectrum. When chloroform is studied in a blend withtoluene, a change in the chemical shift of chloroform is observed, witha greater change observed at higher proportions of toluene in the blend.When chloroform is imbibed into a polymeric particle made of apolymerized units of styrenic monomer, the chemical shift of thechloroform is observed to shift in the same manner as was observed inthe case of the blend of chloroform and toluene. The difference(“DIFF1”) between the chemical shift of chloroform imbibed into thepolymer and the chemical shift of pure chloroform is observed, and it isconcluded that the larger that difference DIFF1 is, the greater theproportion of polymerized units of styrenic monomer are in the immediateregion surrounding the chloroform molecule.

The inhomogeneity of a non-functionalized copolymer comprisingpolymerized units of styrenic monomers may be studied using NMR asfollows. The polymeric bead is swollen with chloroform. In some samples,two different peaks are observed for chloroform, with each peak havingits own chemical shift and therefore its own value of DIFF1. It isconsidered that each peak represents a different region within the bead.It is considered that the peak with larger DIFF1 represents chloroformmolecules surrounded by a relatively larger proportion of polymerizedunits of styrenic monomer, indicating that relatively less chloroformhas imbibed into that region of the particle, in turn indicating thatthat region has higher density of crosslink points. Similarly, it isconsidered that the peak with smaller DIFF1 represents chloroformmolecules located in a region of relatively lower density of crosslinkpoints.

The inhomogeneity of the polymeric bead may also be observed bymicroscopy. For example, optical microscopy and polarized lightmicroscopy each show inhomogeneities that vary with the distance fromthe center of the particle.

A preferred use of the polymer produced in the free radicalpolymerization of the present invention is to be used in a conversionprocess to produce an ion exchange resin. Ion exchange resins fall intothe following categories. Weak base anion exchange resins have pendantamino groups that are primary, secondary, or tertiary. Strong base anionexchange resins have pendant quaternary amino groups. Weak acid cationexchange resins have pendant carboxylic acid groups. Strong acid cationexchange resins have pendant sulfonic acid groups.

Typically, in the preparation of weak base anion exchange resins frompolymeric beads such as crosslinked polystyrene beads, the beads areadvantageously haloalkylated, preferably halomethylated, most preferablychloromethylated, and the ion active exchange groups subsequentlyattached to the haloalkylated copolymer. Typically, the haloalkylationreaction consists of swelling the crosslinked addition copolymer withhaloalkylating agent, preferably bromomethylmethyl ether,chloromethylmethyl ether or a mixture of formaldehyde and hydrochloricacid, most preferably chloromethylmethyl ether and then reacting thecopolymer and haloalkylating agent in the presence of a Friedel-Craftscatalyst such as zinc chloride, iron chloride, or aluminum chloride.Typically, a weak base anion exchange resin is prepared by contactingthe haloalkylated copolymer with ammonia, a primary amine or a secondaryamine. Typically, a strong base anion exchange resin is prepared bycontacting the haloalkylated copolymer with a tertiary amine.

Typically, in the preparation of strong acid cation exchange resins frompolymeric beads such as crosslinked polystyrene beads, the beads areadvantageously sulfonated. Generally, the bead is swollen using asuitable swelling agent and the swollen bead reacted with a sulfonatingagent such as sulfuric acid or chlorosulfonic acid or sulfur trioxide ora mixture thereof.

The inhomogeneous nature of the polymeric bead is preferably stillpresent after sulfonation. This can be observed by NMR as describedabove except that the beads are swollen with water. As in the case ofnon-functionalized polymeric beads swollen with CHCl₃, the molecules ofwater that are imbibed into a functionalized bead of the presentinvention show two peaks, verifying that there are two differentenvironments within the bead of different densities of crosslink points.

Preferably, when the polymeric bead is sulfonated, after the sulfonationprocess the polymeric bead has a relatively even distribution of sulfurbetween the central region of the bead and the outer shell of the bead.This distribution may be assessed as follows. The concentration ofsulfur located in the shell is labeled CSSHELL, and the concentration ofsulfur located in the core is labeled CSCORE. The radial distributionfactor for sulfur, labeled RDFS, is defined as the quotient of CSSHELLdivided by CSCORE:RDFS=CSSHELL/CSCORE

CSCORE and CSSHELL may be characterized by any convenient units, forexample millimole of sulfur per gram of polymer. In practice, becausethe quotient RDFS is the important quantity, the unit used for CSSHELLand CSCORE is not important, as long as the same unit is used for bothCSSHELL and CSCORE. Further, a measurement could be made (such as, forexample, a spectroscopic peak height) (labeled PHSSHELL) that wasproportional to CSSHELL. That is, PHSSHELL=k*CSSSHELL, and the value ofk may not be known. It is contemplated that the same measurement methodcould also produce a result (labeled PHSCORE) that was proportional toCSCORE; that is, PHSCORE=k*CSSHELL. As long as the proportionalityconstant k is the same for both measurements, then the RDFS can bemeasured even though k is unknown, because RDFS=PHSSHELL/PHSCORE.

The quotient RDFS is preferably 0.8 or higher; preferably 0.9 or higher,more preferably 0.95 or higher. The quotient RDFS is preferably 1.2 orlower; more preferably 1.1 or lower; more preferably 1.05 or lower.

It is contemplated that the polymeric beads of the present inventionwould be useful for a variety of purposes. Functionalized polymericbeads would be useful for many of the purposes where ion exchange resinsare useful. Ion exchange resins having improved physical stability—highcrush strength and low degree of breakdown in response to osmoticstress—would be valued by nearly all ion exchange resin end users. Suchresins would be useful in a variety of end use applications, including,for example, water treatment, chromatography, and catalysis. Forexample, in the nuclear industry, there is a specification on crushstrength for ion exchange resins used for water treatment that allsuppliers must meet, and there is a competitive advantage to those whocan supply resins having the highest possible crush strength.

For example, in water treatment in general, the degree of bead breakageon use-regeneration cycles (where the bead changes size due to thedifferent ionic form/water content) will decrease. The reduced beadbreakage will reduce the amount of fine particles mixed with the beads.This will improve resin lifetime and also efficiency and pressure dropfor the end user.

In virtually any uses for ion exchange resins, resistance to mechanicalstress (high crush strength) will minimize bead breakage that couldresults from the mass of beads in an industrial size column, or from themechanical stress applied by pumps, pneumatic conveying systems, etc.

Functionalized polymeric beads made according to the present inventionare preferably cation exchange resins.

For example, the degree of bead breakage on use-regeneration cycles(where the bead changes size due to the different ionic form/watercontent) will decrease. The reduced bead breakage will reduce the amountof fine particles mixed with the beads. This will improve resinlifetime, increase the efficiency of the resin operation, and reduce thepressure drop required to move fluid through a column of functionalizedpolymeric beads.

The following are examples of the present invention.

Raman spectroscopy was performed as follows. The Raman microscopedetects signal from an hour-glass shaped volume. The beam diameter atthe focal point is approximately 1 micrometer and the verticalresolution from the depth of field is approximately 5 to 10 micrometers.The laser was focused down into the sample to a depth of approximately20 micrometers. The beads were swollen in deuterated toluene, which is agood solvent giving a high degree of swelling and good spectralresolution. A Renishaw™ RS-1000 instrument was used to measure thespectra, with a HeNe laser (25 mW, 633 nm), 600 g/mm grating,1064-element TE-cooled CCD detector, and 100×ULWD (ultra-long workingdistance) objective. Spectra at different positions were measuredmanually by stepping across the bead diameter by moving the compressioncell holding the bead.

The beads were swollen fully to equilibrium with the solvent over thecourse of the measurement. The beads were first equilibrated with alarge excess of toluene-D8 (30 mg copolymer and 200 mg toluene-D8) in avial for a few days at room temperature (approximately 23° C.), thenremoved from the solvent and cut in half with a razor blade. The flatsurface was placed face down on the diamond window of a compression cell(Spectra-Tech™) and sealed with a Kalrez™ O-ring. The Kalrez™ O-ringswelled minimally in the toluene solvent. A few drops of toluene-D8 wereadded around the hemispherical bead so that it was soaked in solvent.The top of the diamond compression cell was then attached, and tighteneduntil it just touched the round surface of the bead. The compressioncell was flipped over for the Raman analysis so that the laser could befocused onto the surface of the bead exposed by the cut. The workingdistance of 6 mm of the ULWD objective was just sufficient for theexperiment because of the distance between the stainless steel plate ofthe diamond compression cell and the copolymer sample.

A diagram of the cell is shown in FIG. 2 . The cell is a circularobject, with circular symmetry around the axis 4. The top half 5 of thestainless steel structure and the bottom half 6 of the stainless steelstructure together clamp the O-ring 7. The top half 2 of the stainlesssteel structure and the bottom half 3 of the stainless steel structureare held together by a mechanical arrangement (not shown). The diamondwindows 8 are held in place in the stainless steel structure 5 and 6.The deuterated (D8) toluene 9 partially fills the space enclosed by theO-ring 7, the stainless steel structure 5 and 6, and the diamond windows8. The sample 10 is a bead that is cut in half. The sample 10 is held inplace between the diamond windows 8. The laser beam 11 is focused ontothe sample 10.

Nuclear Magnetic Resonance (NMR) analysis was performed as described byP. J. O'Connor, et al., “1H NMR Characterization of Swelling inCross-Linked Polymer Systems”, Macromolecules Volume 29, Number 24,Pages 7872-7884, 1996; and by Kenji Ogino and Risa.ya Sato, “NMRAnalysis of Interaction Between Styrene-Divinylbenzene Gel Beads andSmall Molecules”, Journal of Polymer Science, Vol. 33, 50 189-195, 1995.

Crush Strength was measured as follows. Functionalized polymeric beadswere placed into contact with air at 100% humidity at 50° C. for 4 days.Then the beads were covered with deionized water and stored for one houror more at room temperature (approximately 23° C.). A single bead wasplaced on one plate of a compression tester at room temperature, and thebead was covered with one drop of water. The plates are brought togetherat 6.0 mm/min until the particle fractures, and the peak force is noted.The procedure is repeated for at least 30 beads, and the average peakforce is reported as the “crush strength.” The test apparatus was aChatillon™ force tester model TCD 200, with a medium-slow motor (2.5 to63.5 mm/min). Force gauge was model DFGS10.

Osmotic stability (OS) was measured as follows. Functionalized polymericbeads were conditioned by contact with a 10% by weight solution of NaClin water for 30 minutes at room temperature (approximately 23° C.). TheNaCl solution was decanted, and the wet resin was passed through meshscreens to produce a sample of resins having diameter of 500 μm to 710μm. Then 4 ml of the resin was placed in a vertical straight-walledglass column internal diameter 10 mm and length of at least 60 mm. Allfluid, when put into the column, was pumped at 1 ml/sec. A single cyclewas as follows: fluid drained from the column by gravity for 56 seconds;resin in the column was contacted with solution #1 for 60 seconds (4seconds to fill the column, 32 seconds of solution #1 passing throughthe column, 4 seconds to fill the column, and 20 seconds of hold timewith solution #1 static in the column); the fluid was drained from thecolumn for 16 seconds, the column was backwashed with water for 10seconds; static water was held in the column for 8 seconds; fluiddrained from the column by gravity for 56 seconds; resin in the columnwas contacted with solution #2 for 60 seconds (4 seconds to fill thecolumn, 32 seconds of solution #2 passing through the column, 4 secondsto fill the column, and 20 seconds of hold time with solution #2 staticin the column); the fluid was drained from the column for 16 seconds,the column was backwashed with water for 10 seconds; static water washeld in the column for 8 seconds. The test was repeated for 50 cycles.Solution #1 was 15% by weight H₂SO₄ in water. Solution #2 was 15% byweight NaOH in water. The cycles of exposure to different solutionscauses some particles to break. After the cycles of exposure, the beadsare placed on a screen (the “500 screen”) that passes objects ofdiameter less than 500 μm. The material passing through the 500 screenis placed on a screen (the “150 screen”) that passes objects of diameterless than 150 μm. The material retained on the 500 screen is consideredwhole beads; this material is dried at 105° C. for 16 hours or more,then cooled to approximately 23° C. in a desiccator, and then weighed;and the weight is reported as W_(whole). The material retained on the150 screen is considered fragments of beads; this material is dried at105° C. for 16 hours or more, then cooled to approximately 23° C. in adesiccator, and then weighed; and the weight is reported as W_(frag).The osmotic stability isOS(%)=100×W _(frag)/(W _(whole) +W _(frag)).Lower OS values are more desirable.

Distribution of sulfur in sulfonated polymeric beads was measured asfollows, using scanning electron microscopy with energy-dispersive x-rayspectroscopy (SEM-EDS). Scanning electron microscopy (SEM) images werecollected with a Hitachi 3400 VP-SEM in variable pressure mode at 15keV. EDX spectra were collected with a Thermo Noran System 6 EDSSpectrometer outfitted with an SDD detector. Radial concentration of Swas measured as follows. Cross sections of the IER were prepared byfracturing with a scalpel; X-Ray spectral maps were collected for thecross sections from at least 4 beads; the net counts (backgroundsubtracted) signal for the characteristic X-Rays from the sulfur wasextracted for a line profile through the center of the bead from theX-Ray spectral map; and the net counts were plotted as a function ofradial distance.

The materials used in the following examples were as followsGelatin=animal based gelatin, isoelectric point approximately 8.5,commercial grade supplied by SOBEL NV (Rousselot)

PADMAC=solution in water (20% by weight) of poly(diallyldimethylammoniumchloride), commercial grade supplied by NALCO

CMMC=carboxymethyl methylcellulose, manufactured by The Dow ChemicalCompany

PVOH=SELVOL™ 523 polyvinylalcohol, from Sekisui Specialty Chemicals

Tris=tris(hydroxymethyl)aminomethane, supplied by Fisher Scientific, 20%by weight solution in water

DVB=divinylbenzene (purity 63% by weight)

BPO=benzoyl peroxide (purity 75% by weight), supplied by Arkema

VPBA=4-vinylphenyl boronic acid, supplied by Beijing Pure Chemical

pbw=parts by weight

caustic=NaOH, 25% by weight solution in water

Polymeric beads were made from approximately 90% styrene by weight andapproximately 10% DVB by weight. Four different recipes were used formaking polymeric beads. During the preparation of the suspension ofmonomer droplets, some individual ingredients or partial mixtures were,if necessary, temporarily heated to achieve good mixing, but thesuspension of monomer droplets was provided at approximately 25° C.Monomer droplets were introduced in the aqueous medium by jetting, asdescribed in U.S. Pat. Nos. 4,444,960 and 4,623,706. In all recipes, theweight ratio of droplet ingredients to aqueous phase ingredients was0.61:1.

The recipes are summarized as follows. “Comp.” means comparative.“Addition of Base” is the extent of conversion of monomer to polymer(approximate) at which base was added.

Addition pH at pH at Example Replicate Aqueous Phase of Base start endComp. 1 A Gelatin, none 10.2 9.5 PADMAC Comp. 1 B Gelatin, none 10.2 9.5PADMAC Comp. 2 A CMMC only 43% and 84% 6.8 11.5 Comp. 2 B CMMC, PVOH,40% and 83% 6.4 10.4 VPBA 3 A PVOH, VPBA 82% 6.0 7.5 3 B PVOH, VPBA 82%6.0 7.5 4 A PVOH VPBA none 6.1 4.5 4 B PVOH VPBA none 6.1 4.7

Comparative Examples 1A and 1B did not begin polymerization at pH of 7or less. Comparative Examples 2A and 2B had two additions of base thatraised the pH, and one of these additions was made at monomer conversionof 43% and 40% (i.e., prior to 60%). The details of the methods ofmaking the polymeric beads were as follows:

COMPARATIVES EXAMPLE 1A and 1B Gelatin/PADMAC, High pH

Droplet composition was as follows: (% by weight based on the totalweight of droplet ingredients:

Styrene DVB BPO balance 10.15% 0.3%

Aqueous medium composition was as follows (% by weight based on totalweight of aqueous medium):

DI Water PADMAC Gelatin NaOH Boric Acid NaNO₂ balance 2.03% 0.23% 0.11%0.17% 0.016%

Aqueous suspension polymerization was conducted on the suspension ofmonomer droplets as follows. The suspension was heated to 72° C., andthe extent of reaction was monitored. Once conversion to polymer was inthe 80-85% range, the suspension was heated to 92° C. The pH began at10.2 and finished at 9.5. After 1 hour, the suspension was cooled toambient temperature and the beads were dewatered, washed with water, anddried at ambient temperature.

Sulfonation was performed as follows. 100 pbw of polymeric beads, 449pbw of a solution of 96% by weight sulfuric acid in water, 5.2 parts byweight of DI water, and 35 pbw of EDC were added one by one to a reactorat ambient temperature, and heated to 130° C. over a 135 minute period.The system was held at 130° C. for 210 minutes, then cooled to 100° C.Once at 100° C., a hydration process was started, in order to removeacid and introduce water. The hydration was done in stages, using a 50%acid cut, a 25% acid cut, and then water. The hydration fluids are addedat ambient temperature, so the batch temperature drifts from 100° C. toambient temp over the course of the hydration. Following hydration, thewater-swollen polymeric beads were washed with ambient temperaturewater, then with 98° C. water, then ambient water again. The excesswater was filtered, isolating the water-swollen polymeric beads.

Duplicate samples were made, labeled 1A and 1B.

COMPARATIVE EXAMPLE 2A CMMC Only; NaOH Additions at 43% and 84%

Droplet composition was as follows: (% by weight based on the totalweight of droplet ingredients:

Styrene DVB BPO balance 9.60% 0.3%

Aqueous medium composition was as follows (% by weight based on totalweight of aqueous medium):

DI Water CMMC NaNO₂ balance 0.15% 0.016%

Aqueous suspension polymerization was conducted as follows. Thesuspension was heated to 83° C. for 90 minutes and then cooled to 80°Cover a 10 minute period, and the extent of reaction was monitored.Caustic was added when the extent of reaction was in the 40-45% range,and when extent of reaction (conversion) was in the 80-85% range. Onceconversion to polymer was in the 80-85% range, the reaction system washeated to 92° C. After 1 hour, the system was cooled to ambienttemperature and the beads were dewatered, washed with water, and driedat ambient temperature. The polymeric beads were sulfonated using thesame procedure as Comparative Example 1A.

Comparative Example 2B (CMMC/PVOH/VPBA; NaOH additions at 42% and 85%conversion).

Monomer droplet composition was as follows: (% by weight based on thetotal weight of droplet ingredients):

Styrene DVB VPBA BPO balance 9.60% 0.01% 0.3%

Aqueous medium composition was as follows (% by weight based on totalweight of aqueous medium):

DI Water CMMC PVOH NaNO₂ balance 0.08% 0.06% 0.016%

Aqueous suspension polymerization was conducted as follows. Thesuspension was heated to 83° C. for 90 minutes and then cooled to 80° C.over a 10 minute period, and the extent of reaction was monitored.Caustic was added when the extent of reaction was in the 40-45% range,and when extent of reaction (conversion) was in the 80-85% range. Oncethe extent of reaction was in the 80-85% range, the reaction system washeated to 97° C. After 1 hour, the system was cooled to ambienttemperature, and the beads were dewatered, washed with water, and driedat ambient temperature. The polymeric beads were sulfonated using thesame procedure as Comparative Example 1A.

EXAMPLE 3A AND 3B PVOH/VPBA; Tris Addition at 82% Conversion

Monomer droplet composition was the same as in Comparative Example 2B.The aqueous medium composition was as follows (weight % based on theweight of the aqueous composition):

DI Water PVOH NaNO₂ balance 0.06% 0.016%

Aqueous suspension polymerization was conducted as follows. Thesuspension was heated to 83° C. for 90 minutes and then cooled to 80° C.over a 10 minute period, and the extent of reaction was monitored. Triswas added when extent of reaction (conversion) was in the 80-85% range.Once the extent of reaction was in the 80-85% range, the reaction systemwas heated to 97° C. After 1 hour, the system was cooled to ambienttemperature and the beads were dewatered, washed with water and dried atambient temperature. The polymeric beads were sulfonated using the sameprocedure as Comparative Example 1A. Duplicate samples were made,labeled 3A and 3B.

EXAMPLE 4A PVOH/VPBA; No pH Adjustment

Monomer droplet composition and aqueous medium composition were the sameas in Example 3A. Aqueous suspension polymerization was conducted asfollows. The suspension was heated to 83° C. for 90 minutes and thencooled to 80° C. over a 10 minute period, and the extent of reaction wasmonitored. Once the extent of reaction was in the 80-85% range, thereaction system was heated to 92° C. After 1 hour, the system was cooledto ambient temperature and the beads were dewatered, washed with waterand dried at ambient temperature. The polymeric beads were sulfonatedusing the same procedure as Comparative Example 1A.

EXAMPLE 4B PVOH/VPBA; No pH Adjustment

Monomer droplet composition was as follows: (% by weight based on thetotal weight of droplet impediments):

Styrene DVB VPBA BPO balance 9.60% 0.01% 0.3%

The composition of the aqueous medium, the polymerization procedure, andthe sulfonation procedure were the same as in Example 4A.

Results of Raman Spectroscopy of Polymeric Beads: C═C double bondstretching.

Samples were prepared and Raman spectra were obtained at various pointsalong a diameter of a bead as described above. PCC was the height of theRaman peak at 1635 cm⁻¹ due to stretching of carbon-carbon double bonds.PAR was the height of the Raman reference peak at 1000 cm⁻¹ due tostretching of the aromatic ring. The quotient V1=PCC/PAR characterizesthe prevalence of double bonds within the polymer. FIG. 3 shows a plotof V1 versus position for Comparative Example 1B. FIG. 4 shows a plot ofV1 versus position for Example 4B.

FIG. 4 shows that Example 4A has relatively high values of V1 at theedges of the bead in comparison to the middle of the bead. This meansthat Example 4A has a relatively high proportion of unreacted doublebonds near the circumference of the bead in comparison to the middle ofthe bead.

In referring to FIGS. 3 and 4 , “distance from the center of the bead”is considered herein to be an absolute value, regardless of thedirection from the center of the bead. Thus a point having a value onthe horizontal axis of negative 0.9 is considered to have distance fromthe center of 0.9*R, the same as a point at a value on the horizontalaxis of positive 0.9.

The relative peak heights near the circumference of the bead as comparedto the peak heights near the center of the bead were assessed asfollows. The values of V1 were averaged for points at radial distancefrom the center of the bead of 0.8*R to R, and this average was labeledV1SHELL. Similarly, the values of V1 were averaged for points at radialdistance from the center of the bead of 0 to 0.5*R, and this average waslabeled V1CORE. Then the quotient is calculated:VR=V1SHELL/V1COREwhich is the radial distribution factor for unreacted vinyl groups. Itis contemplated that curves like FIG. 3 will have relatively low valueof VR, while curves like FIG. 4 will have relatively high value of VR.

Using the Raman results, a similar assessment can be made using theratio of the CD stretch to the CH stretch to yield RDFS, the radialdistribution factor for swelling solvent.

In order to assess crush strength and osmotic stability, the polymericbeads were functionalized (i.e., had sulfonic acid groups attached) asdescribed above.

The results of the testing were as follows:

Crush O.S. Example VR RDFS (g) (% break) Comp. 1A 0.18 1.07 518 23.2Comp. 1B 1.19 1.63 531 14.6 Comp. 2A 2.06 1.62 1064 1.0 Comp. 2B 2.022.29 987 2.6 3A 4.12 2.97 1888 0.4 3B 4.08 4.50 1957 0.3 4A 5.28 4.632606 0.2 4B 5.81 2.65 2893 0.2The Example polymers 3A, 3B, 4A, and 4B were all made by the method ofthe present invention, and all had VR greater than 3.5, while thecomparative polymers had VR less than 2.1. The Example polymers also allhad RDFS greater than 2.6, while the comparative polymers had RDFS lessthan 2.3. The Example polymers showed superior performance properties(i.e., crush strength and osmotic stability) compared to the comparativepolymers.

Results of SEM-EDS testing of sulfonated polymeric beads. The beadstested were sulfonated resins made from the copolymers of ComparativeExamples 2A and 2B and of Example 4A. In all samples, the SEM-EDS testresults in an image that shows the presence of sulfur in a cross sectionof the bead through the center. Visual inspection of these images showedthat sulfur is evenly distributed throughout the cross section of theparticle. Also, the image may be analyzed digitally to produce a graphof sulfur content as a function of position along a diameter line of thecross section. Such a graph shows the sulfur content to be constantalong the diameter line. These results shown that the distribution ofsulfur is uniform throughout the bead. Thus any improved properties inthe beads of the present invention are not due to any inhomogeneities inthe distribution of sulfonic acid groups in the bead.

Results of NMR analysis on non-functionalized polymeric beads were asfollows. The solvent was CHCl₃. When a comparative homogeneous bead wastested, one peak was observed at approximately 6.7 ppm, corresponding tosolvent swollen into the bead, and a second peak was observed atapproximately 7.2 ppm, corresponding to free solvent. When a sample ofthe present invention was tested, a free solvent peak at 7.2 ppm wasobserved as in the comparative sample, but in the inventive sample, thepeak corresponding to solvent swollen into the bead was split into twopeaks, one higher than 6.7 ppm and one lower than 6.7 ppm. It isconsidered that the two peaks above and below 6.7 ppm demonstrate thatthe imbibed solvent is present in two different environments. It isconsidered that one environment is the relatively highly crosslinkedcore and the other environment is the relatively lightly crosslinkedshell.

Results of NMR analysis of sulfonated polymeric beads were as follows.The solvent was water. In the comparative sample, which is homogeneous,a free water peak was observed at approximately 4.7 ppm, and a peak atapproximately 6.4 ppm due to water imbibed into the bead was observed.When a polymeric bead of the present invention was tested, the same freewater peak at 4.7 ppm was observed, but the single peak at 6.4 ppm wasreplaced by a pair of peaks, one at lower ppm than 6.4 ppm and onehigher than 6.4 ppm. It is considered that the two peaks above and below6.4 ppm demonstrate that the imbibed solvent finds itself in twodifferent environments. It is considered that one environment is therelatively highly crosslinked core and the other environment is therelatively lightly crosslinked shell.

The invention claimed is:
 1. A process of making a polymeric beadcomprising (a) providing a suspension of monomer droplets in an aqueousmedium at pH of 7 or less, wherein the monomer droplets comprise one ormore monofunctional vinyl monomers, one or more multifunctional vinylmonomers, and one or more initiators, wherein the aqueous mediumcomprises one or more derivatives of a nitrite salt in an amount, byweight based on the weight of the aqueous medium, of 0.005% to 0.5%, and(b) initiating polymerization of the monomer, wherein no pH-raisingsubstance is added after beginning step (b) until 60% or more by weightof all monofunctional monomer has been converted to polymer; wherein thepolymeric bead has a radius R; wherein the polymeric bead comprises 0.3%to 20% by weight, based on the weight of the polymeric bead, ofpolymerized units of the one or more multifunctional vinyl monomers and80% to 99.7% by weight, based on the weight of the polymeric bead, ofpolymerized units of the one or more monofunctional vinyl monomers;wherein one or more of the monofunctional vinyl monomers or one or moreof the multifunctional vinyl monomers comprises an aryl group; and (i)wherein the polymerized units of multifunctional vinyl monomer haveradial distribution factor MR of 0.9 to 1.1, wherein MR=CMSHELL/CMCORE,wherein CMSHELL is the average concentration of polymerized units ofmultifunctional vinyl monomer located at a distance from the center ofthe bead of 0.8*R to R, and wherein CMCORE is the average concentrationof polymerized units of multifunctional vinyl monomer located at adistance from the center of the bead of 0 to 0.5*R, and (ii) whereinsome of the vinyl groups in the polymerized units of multifunctionalvinyl monomer are unreacted, and the unreacted vinyl groups have aradial distribution factor VR of 2.5 or higher, wherein VR is determinedby a Raman spectroscopic measurement performed on the bead, whereinVR=V1SHELL/V1CORE, wherein V1SHELL is the average of ratio V1 formeasurements made at a distance from the center of the bead of 0.8*R toR, wherein V1CORE is the average of ratio V1 for measurements made at adistance from the center of the bead of 0 to 0.5*R, wherein V1=PCC/PAR,wherein PCC is the height of the Raman spectroscopic peak due tostretching of carbon-carbon double bonds, and PAR is the height of theRaman spectroscopic reference peak due to stretching of the aryl groupat 1000 cm⁻¹.
 2. The process of claim 1, wherein the polymerization is asingle-step polymerization.
 3. The process of claim 1, wherein themonomer droplets, prior to step (b), either contain no polymer or elsecontain polymer in an amount less than 10% by weight based on the weightof the monomer droplets.
 4. The process of claim 1, wherein the monomerdroplets, prior to step (b), either contain no porogen or else containporogen in an amount less than 10% by weight based on the weight of themonomer droplets.
 5. The process of claim 1, further comprising thestep, after completion of the polymerization, of sulfonating thepolymeric beads.
 6. The process of claim 1, wherein the monofunctionalvinyl monomers comprise one or more styrenic monomer.
 7. The process ofclaim 1, wherein the multifunctional vinyl monomers comprise one or morestyrenic monomer.
 8. The process of claim 1, wherein the polymeric beadcomprises 8% to 15% by weight, based on the weight of the polymericbead, of polymerized units of one or more multifunctional vinyl monomerand 85% to 92% by weight, based on the weight of the polymeric bead, ofpolymerized units of one or more monofunctional vinyl monomer.
 9. Theprocess of claim 1, wherein the polymer bead has a volume averageparticle diameter of 50 μm or larger.
 10. The process of claim 1,wherein the polymer bead has a volume average particle diameter of 100μm or larger.
 11. The process of claim 1, wherein the polymer bead has avolume average particle diameter of 200 μm or larger.
 12. The process ofclaim 1, wherein the polymer bead has a volume average particle diameterof 400 μm or larger.
 13. The process of claim 1, wherein the polymerbead has a volume average particle diameter ranging from 50 μm to 1,500μm.
 14. The process of claim 1, wherein the polymer bead has a volumeaverage particle diameter ranging from 100 μm to 1,000 μm.
 15. Theprocess of claim 1, wherein the polymer bead has a volume averageparticle diameter ranging from 200 μm to 1,000 μm.
 16. The process ofclaim 1, wherein the polymer bead has a volume average particle diameterranging from 400 μm to 1,000 μm.
 17. The process of claim 1, wherein themonofunctional vinyl monomers comprise styrene.
 18. The process of claim1, wherein the multifunctional vinyl monomers comprise divinylbenzene.19. The process of claim 1, wherein the monofunctional vinyl monomerscomprise styrene and the multifunctional vinyl monomers comprisedivinylbenzene.
 20. The process of claim 1, wherein the aqueous mediumcomprises one or more derivatives of a nitrite salt in an amount, byweight based on the weight of the aqueous medium, of 0.014% to 0.2%.